What sets the transmission strength of synapses? What determines their plasticity properties? How do synapses homeostatically adjust synaptic weight to accommodate to changing conditions and ensure robust behavior? What happens if synaptic homeostasis is insufficient to compensate for a disruption or a change in demand, are other backup mechnisms of compensation recruited and, if so, how do they work? We combine in vivo super-resolution quantal imaging of synaptic transmission and behavioral analysis with focused RNAi knockdown in one cell type and single cell transcriptome analysis to address these questions. Our preparation is the Drosophila larval neuromuscular junction?an ideal system for imaging and genetics, which shares synaptic signaling machinery and functional properties with vertebrate central excitatory synapses. Our in vivo quantal analysis has revealed that two converging glutamatergic motor neuron (MN) inputs have great heterogeneity in evoked release probability (Pr) and short-term plasticity and that only Ib undergoes ?synaptic homeostasis,? whereby transmitter release changes to compensate for altered postsynaptic sensitivity. Our goal is to identify the molecules responsible for the synapse to synapse and input to input differences. Equally exciting, preliminary work suggests the existence of a novel layer of gain control: ?circuit homeostasis,? which is recruited when synaptic transmission is so compromised that ?synaptic homeostasis? cannot compensate sufficiently. The circuit homeostasis system adjusts neural firing pattern in the presynaptic cell and upstream circuit to preserve locomotor behavior when synaptic transmission is inadequate. Our goal is to define the mechanisms that assure neural output by setting and adjusting transmitter release and firing dynamics. Progress will provide fundamental insight into the robustness of the nervous system that preserves health and which may cause disease when it goes awry.